In general, the choice of an appropriate method and means for monitoring mass-produced radio-receiving equipment is a tradeoff between the tendency to reduce the monitoring time and the desire to increase the objectiveness of the results and the control range. One method to overcome this problem is an automated technique of measuring receiver parameters.
Known in the art is a broadcast receiver monitoring system /cf. "Radioelectronics Review", Series TRPA, issue 2, 1975, pp. 27-31/ comprising frequency synthesizes generating an input test signal, a set of electronically controlled instruments, and a mechanical manipulator performing necessary operations with the control members of the receivers to be tested. All the abovementioned devices are program-controlled using a computer. This technique provides for a fully automatic monitoring system, but it suffers from a number of disadvantages including a relatively long monitoring time, as the receiver parameters are measured successively; also the receiver has to be tuned to the test signal frequency. A further shortcoming resides in a complexity and high cost of the monitoring equipment. In addition, the use of the mechanical manipulator results in an inflexible system and necessitates a test operator to perform the job, because the mechanical manipulator does not make it possible to detect some faults in the receiver such as the tuning knob play, the vernier pointer sticking, etc.
The above disadvantages are overcome in an apparatus for automatic monitoring of superheterodyne radio receivers as disclosed in U.S. Pat. No. 3,755,741 published in 1973. This apparatus comprises a unit for shaping the receiver tune-in frequency signal, and a shaper of the desired signal for modulating the tune-in frequency signal. The tune-in frequency shaping unit is composed of a master oscillator for generating the voltage at a frequency equal to the intermediate frequency of the receiver, a mixer, a filter, and a modulator, all connected in series, the second input of the mixer being connected to the output of the heterodyne of the receiver to be tested, and the modulating input of the modulator connected to the desired signal shaper formed by an audio-frequency sine-wave oscillator. The output of the modulator is connected to the input of the receiver being tested, while the output of the receiver is connected to an indicating unit.
In this prior art device, the test signal applied to the input of the receiver results from modulating its heterodyne frequency of the intermediate frequency followed by the amplitude modulation of the FM signal by the audio frequency voltage, resulting in the test signal frequency being invariably equal to the receiver tune-in frequency, so that the process of tuning the receiver to the test signal frequency is omitted, thus reducing the monitoring time. In addition, the device is relatively simple in design and, consequently, not expensive.
The apparatus according to the aforementioned U.S. Patent, however, enables only two receiver parameters to be controlled, i.e. the sensitivity and the harmonic distortion factor at a single audio frequency, since the test signal represents an r.f. voltage at a single carrier frequency, amplitude modulated by a low-frequency voltage also at a single frequency. Therefore, the receiver monitoring proves to be not valid, since it is impossible to evaluate the quality of the receiver from two parameters.
Another disadvantage of the apparatus described lies in a complicated FM scheme outside the master oscillator if FM signal receivers are to be tested.